1. Field of the Invention
The present invention relates to an electronic device mounted with a printed circuit board, and a method for manufacturing the same. Moreover, the present invention relates to a method for shielding the printed circuit board in order to reduce electromagnetic noises radiated from the printed circuit board.
2. Description of Related Art
In earlier technology, in an electronic device mounted with a printed circuit board, as shown in FIG. 7A, a shielding plate 30 is disposed in only one face side of a printed circuit board 10, or as shown in FIG. 7B, a printed circuit board 20 is installed into a box-like shielding member 40 and all the six faces of the printed circuit board 20 are shielded. The shielding plate 30 and the shielding member 40 corresponding to the printed circuit boards 10 and 20, respectively, are used in order to prevent electromagnetic wave from radiating from the wiring patterns (not shown) in the printed circuit boards 10 and 20. The shielding plate 30 and the shielding member 40 are away from the printed circuit boards 10 and 20 for a predetermined distance, respectively.
Accordingly, air exists between the shielding plate 30 and the printed circuit board 10 or the shielding member 40 and the printed circuit board 20, so that each dielectric constant is low, and the high frequency loss is small. Further, their structures are for shutting out the noises generated in the inside of the printed circuit boards 10 and 20 and the noises radiated from the outside of the boards 10 and 20. It was not particularly necessary to connect the printed circuit boards 10 and 20 to grounds.
Such shielding structures have shielding effects to the noises radiated from the printed circuit boards 10 and 20 themselves, that is, the noises radiated from the printed circuit boards 10 and 20 toward the shielding plate 30 and the shielding member 40, respectively. However, generally, in a printed circuit board, there is a cable for connecting to the outside. Thereby, even though the printed circuit board is shielded, if this cable is connected to the outside of the shield, noises (there are two noises, a common mode noise such that the noises of the power supply and that of the ground are common mode, and a differential mode noise such that those noises appear in differential mode) appear in the power supply and the ground (GND) itself of the printed circuit board, and this cable will operate as an antenna and will generate a big noise. That is, with the structure of such a shielding plate or a shielding member in earlier technology, the effect of suppressing the high frequency noise generated in the ground plane of a printed circuit board cannot be expected. Therefore, there were few effects of reducing a noise to the printed circuit board connected with a cable.
Further, since the distance between the wiring patterns of the printed circuit boards 10 and 20 and the shielding plate 30 and the shielding member 40, respectively, is large, particularly in the case of a double-sided board, the effect of the shielding plate 30 or the shielding member 40 as a return path of signals which flow the wiring patterns of the printed circuit boards 10 and 20 cannot be expected, either. Therefore, reinforcement of the ground plane of the printed circuit board is required for the countermeasure against noise. However, in order to reinforce the ground plane of a printed circuit board having a high mounting density of the parts, it is necessary to use quite a high multi-layer printed circuit board. Further, it is necessary to connect between the power supply and the GND by disposing many decoupling capacitors having good characteristic in order to reduce the noise between the power supply and the GND. Here, in the specification, the ground may be indicated as GND, as mentioned above.
As a solution for such a problem, as disclosed in the Japanese Patent Laid-Open Publication No. 5-500136 (Applicant, Xeikon corporation), a technology such that the noise generated between the power supply and the GND is absorbed by the printed circuit board itself and reduced by forming a thin layer having a high dielectric constant between the power supply layer and the GND layer of a multi-layer printed circuit board is suggested. However, it is required to use a special material, so that it has not been used widely in general in respect of cost or availability.
An object of the present invention is to suppress a common mode noise or a differential mode noise generated between a ground layer (GND layer) and a power supply layer.
Further, another object of the present invention is to reduce a noise generated from a printed circuit board without making the printed circuit board high multi-layered so much.
In order to achieve the above-described objects, according to a first aspect of the present invention, the electronic device comprises: a printed circuit board having a ground; an electrically insulating layer provided on a face of the printed circuit board; and an electromagnetic shielding layer adhered to the face of the printed circuit board through the electrically insulating layer. The ground of the printed circuit board and the electromagnetic shielding layer are conducted electrically.
The xe2x80x9cprinted circuit boardxe2x80x9d is a general term of a board for mounting electronic circuit parts thereon, including a printed wiring board, a flexible printed circuit board and the like. Further, the xe2x80x9celectronic devicexe2x80x9d is an electronic device mounted with a printed circuit board, such as a copying machine, a printer, a personal computer, a measuring device and the like.
Further, in this electronic device, the electrically insulating layer may be a resin layer, and the electromagnetic shielding layer may be formed by applying an electroconductive paste on the resin layer. Then, a coat may be provided on the electromagnetic shielding layer formed with the electroconductive paste.
Moreover, the electromagnetic shielding layer is a layer having electroconductivity. It may be plate-like, and furthermore, it may be a metal member. Here, the xe2x80x9cmetal memberxe2x80x9d includes a metal frame, a metal plate, a metal panel and the like of the electronic device mounted with the printed circuit board.
Preferably, the electromagnetic shielding layer covers an end face of the printed circuit board.
Further, the printed circuit board may be a multi-layer printed circuit board in which a power supply layer is disposed in an external layer of the electromagnetic shielding layer side.
Moreover, the electromagnetic shielding layer may be a bag-like member, and the electrically insulating layer may be provided on an inner face of the bag-like member.
Preferably, the electrically insulating layer is sheet-like, and more preferably, a dielectric constant of the electrically insulating layer is not less than 5. Further, the electrically insulating layer may be provided previously on the printed circuit board, or may be provided separately on the printed circuit board.
According to a second aspect of the present invention, the method for manufacturing an electronic device, comprises: providing an electrically insulating layer on a face of a printed circuit board having a ground; providing an electromagnetic shielding layer so as to adhere to the face of the printed circuit board through the electrically insulating layer; and conducting the ground of the printed circuit board and the electromagnetic shielding layer electrically.
Further, in the method for manufacturing the electronic device, the providing the electrically insulating layer may comprise applying an electrically insulating resin layer, and the providing the electromagnetic shielding layer may comprise applying an electroconductive paste on the resin layer.
Preferably, the method further comprises providing a coat on the electromagnetic shielding layer provided with the electroconductive paste.
According to a third aspect of the present invention, the method for manufacturing an electronic device, comprises: inserting a printed circuit board into a bag-like electromagnetic shielding member, an electrically insulating layer being provided on an inner face of the electromagnetic shielding member; and adhering the electromagnetic shielding member to a face of the printed circuit board through the electrically insulating layer by extracting air in the bag-like electromagnetic shielding member in which the printed circuit board is inserted.
According to a fourth aspect of the present invention, the method for shielding a printed circuit board, comprises: providing an electrically insulating layer on a face of the printed circuit board having a ground; providing an electromagnetic shielding layer so as to adhere to the face of the printed circuit board through the electrically insulating layer; and conducting the ground of the printed circuit board and the electromagnetic shielding layer electrically.
According to a fifth aspect of the present invention, the method for shielding a printed circuit board, comprises: inserting a printed circuit board into a baglike electromagnetic shielding member, an electrically insulating layer being provided on an inner face of the electromagnetic shielding member; and adhering the electromagnetic shielding member to a face of the printed circuit board through the electrically insulating layer by extracting air in the bag-like electromagnetic shielding member in which the printed circuit board is inserted.
According to the electronic device and the method for shielding the printed circuit board of the present invention, since the ground of the printed circuit board and the electromagnetic shielding layer or the electromagnetic shielding member are electrically conducted, the electromagnetic shielding layer or member functions not only as a shield for shielding an electromagnetic noise radiated from the printed circuit board but also as a ground layer. That is, the same effect as increasing one ground layer can be obtained. Therefore, a return path of signals from the wiring patterns of the printed circuit board can be obtained stably. Thus, the ground can be reinforced without making the printed circuit board high multi-layered.
Further, since the electrically insulating layer is sandwiched in between the printed circuit board and the electromagnetic shielding layer or member which functions as a ground layer, the capacity of the capacitor formed between the signal layer of the printed circuit board and the ground layer, between the power supply layer and the ground layer, or the like, becomes large. Thereby, the characteristic impedance of the printed circuit board lowers, and noises are absorbed by the whole printed circuit board. In particular, the noises (common mode noise and differential mode noise) generated between the power supply layer and the ground layer can be reduced well. Moreover, the capacity of the above-described capacitor can be made large, and the above-described return path can also be made short by adhering the electromagnetic shielding layer or member to at least one face of the printed circuit board through the electrically insulating layer. Therefore, noises can be further reduced.
Thus, noises, particularly the noises generated between the power supply layer and the ground layer can be reduced well without making the printed circuit board high multi-layered.
Further, according to the method for manufacturing the electronic device of the present invention, an electronic device by which the above-described effects are obtained can be manufactured.